Fuel cells can be used as a power source in many applications. For example, fuel cells have been proposed for use in automobiles as a replacement for internal combustion engines. In proton exchange membrane (PEM) type fuel cells, a reactant such as hydrogen is supplied as a fuel to an anode of the fuel cell, and a reactant such as oxygen or air is supplied as an oxidant to the cathode of the fuel cell. The PEM fuel cell includes a membrane electrode assembly (MEA) having a proton transmissive, non-electrically conductive, proton exchange membrane. The proton exchange membrane has an anode catalyst on one face and a cathode catalyst on the opposite face. The MEA is often disposed between “anode” and “cathode” diffusion media or diffusion layers that are formed from a resilient, conductive, and gas permeable material such as carbon fabric or paper. The diffusion media serve as the primary current collectors for the anode and cathode as well as providing mechanical support for the MEA and facilitating a delivery of the reactants.
In a fuel cell stack, a plurality of fuel cells is aligned in electrical series, while being separated by gas impermeable, electrically conductive bipolar plates. Each MEA is typically sandwiched between a pair of the electrically conductive plates that serve as secondary current collectors for collecting the current from the primary current collectors. The plates conduct current between adjacent cells internally of the fuel cell stack in the case of bipolar plates and conduct current externally of the stack in the case of unipolar plates at the ends of the stack.
The bipolar plates typically include two thin, facing conductive sheets. One of the sheets defines a flow path on one outer surface thereof for delivery of the fuel to the anode of the MEA. An outer surface of the other sheet defines a flow path for the oxidant for delivery to the cathode side of the MEA. When the sheets are joined, a flow path for a dielectric cooling fluid is defined.
The typical bipolar plate is a joined assembly constructed from two separate unipolar plates. Each unipolar plate has an exterior surface having flow channels for the gaseous reactants and an interior surface with the coolant channels. The bipolar plates have a complex array of grooves or channels that form flow fields for distributing the reactants over the surfaces of the respective anodes and cathodes. Tunnels are also internally formed in the bipolar plate and distribute appropriate coolant throughout the fuel cell stack, in order to maintain a desired temperature.
The separate unipolar plates are typically produced from a formable metal that provides suitable strength, durability, rigidity, electrical conductivity, and corrosion resistance, such as 316 alloy stainless steel, for example. Austenitic stainless steels have been successfully formed by various processes such as, for example, machining, molding, cutting, carving, stamping, or photo-etching, into bipolar plate materials for PEM fuel cells. The austenitic stainless steel exhibits high corrosion resistance due to a thin passive oxide film on the surface thereof. However, the thin passive oxide film undesirably increases the contact resistance between the bipolar plate surface and the gas diffusion media (GDM) adjacent thereto. To maximize fuel cell performance and current densities, it is desirable to reduce fuel cell resistances. Reducing the contact resistance between the bipolar plate surface and the GDM can significantly reduce total fuel cell resistance, thereby improving performance and current density.
It is known to mitigate high contact resistance by coating stainless steel bipolar plates with expensive noble metals, such as gold, to obtain a low contact resistance between the bipolar plate surface and the GDM. Alternatively, it is known that iron, and to a lesser extent chromium, enrichment in the passive oxide film of a stainless steel alloy increases, rather than decreases, the contact resistance between the bipolar plate surface and the GDM. It is also known that coating the bipolar plates with a high-nickel-content alloy or carbon achieves a significant reduction of the contact resistance between the GDM and the bipolar plate, and would eliminate the need for expensive noble-metal coatings that are currently being used. However, such coatings are not sufficiently durable to withstand stamping or other manufacturing processes.
Additionally, conventional processes of forming the plates from the metal sheet material result in nearly half of the material being discarded as scrap. Some of the scrap is generated as apertures are punched in the non-active portion of the plates to create flow areas and manifolds for delivery and exhaust of reactants and coolant when a plurality of bipolar plates is aligned in the fuel cell stack. A larger portion of the scrap results from a clamping area that is required about the perimeter of the sheet material during the processes that form plates from the sheet material, which is then trimmed or cut off after processing.
There is a continuing need for a cost-effective bipolar plate assembly having an efficient and robust structure that provides an optimized electrical contact between the plates of the assembly while minimizing material usage and waste and maximizing the structural integrity of the plates. A method for rapidly producing the bipolar plate assembly applicable to optimized flowfield designs is also desired.